Theories of a massive graviton are promising candidates to
explain the accelerating expansion of the Universe, in addition to
holding significant intrinsic theoretical interest. The bimetric theory
of massive gravity in particular has a wealth of cosmological solutions
which could describe our Universe.
In the first half of the talk, I will
introduce bimetric massive gravity and its cosmologies.
In the second
half, I will discuss the subtleties associated with understanding
cosmological perturbation theory in bigravity due to the complicated
nonlinear behavior of the helicity-0 mode of the massive graviton.